1 JDBC – Java Database Connectivity Representation and Management of Data on the Internet

2 Introduction to JDBC JDBC is used for accessing databases from Java applications Information is transferred from relations to objects and vice-versa –databases optimized for searching/indexing –objects optimized for engineering/flexibility

3 JDBC Architecture Java Application JDBC Oracle DB2 Postgres Oracle Driver DB2 Driver Postgres Driver These are Java classes Network We will use this one…

JDBC Architecture (cont.) ApplicationJDBCDriver Java code calls JDBC library JDBC loads a driver Driver talks to a particular database Can have more than one driver -> more than one database Ideal: can change database engines without changing any application code

5 Seven Steps Load the driver Define the connection URL Establish the connection Create a Statement object stmt Execute a query using stmt Process the result Close the connection

6 Loading the Driver We can register the Driver indirectly using the Java statement: Class.forName(“oracle.jdbc.driver.OracleDriver"); Calling Class.forName causes the Driver class to be loaded When this class is loaded, it automatically –creates an instance of itself –registers this instance with the DriverManager

7 Another Option Another option is to create an instance of the driver and register it with the Driver Manager: Driver driver = new oracle.jdbc.OracleDriver(); DriverManager.registerDriver(driver);

8 An Example // A driver for imaginary1 Class.forName("ORG.img.imgSQL1.imaginary1Driver"); // A driver for imaginary2 Driver driver = new ORG.img.imgSQL2.imaginary2Driver(); DriverManager.registerDriver(driver); //A driver for oracle Class.forName("oracle.jdbc.driver.OracleDriver"); imaginary1 imaginary2 Registered Drivers Oracle

9 Connecting to the Database Every database is identified by a URL Given a URL, DriverManager is asked to find the driver that can talk to the corresponding database DriverManager tries all registered drivers, until a suitable one is found

10 Connecting to the Database Connection con = DriverManager. getConnection("jdbc.imaginaryDB1"); imaginary1 imaginary2 Registered Drivers Oracle    acceptsURL(“jdbc.imaginaryDB1”)? Read more in DriverManager APIDriverManager API

11 The URLs in CS In CS, a URL has the following structure: For example: A complete example Your login Also, your login The machine on which our DBMS runs The standard port given to Oracle on sol4

Interaction with the Database We use Statement objects in order to –Extract data from the database –Update the database Three different interfaces are used: Statement, PreparedStatement, CallableStatement All are interfaces, thus cannot be instantiated They are created by the Connection

13 Querying with Statement The executeQuery method returns a ResultSet object representing the query result. Will be discussed later… String queryStr = "SELECT * FROM Member " + "WHERE Lower(Name) = 'harry potter'"; Statement stmt = con.createStatement(); ResultSet rs = stmt.executeQuery(queryStr);

14 Changing DB with Statement String deleteStr = “DELETE FROM Member " + "WHERE Lower(Name) = ‘harry potter’"; Statement stmt = con.createStatement(); int delnum = stmt.executeUpdate(deleteStr); executeUpdate is used for data manipulation: insert, delete, update, create table, etc. (anything other than querying!) executeUpdate returns the number of rows modified

15 About Prepared Statements Prepared Statements are used for queries that are executed many times They are parsed (compiled) by the DBMS only once Column values can be set after compilation Instead of values, use ‘?’ Hence, a Prepared Statement is statement that contains placeholders to be substituted later with actual values

16 Querying with PreparedStatement String queryStr = "SELECT * FROM Items " + "WHERE Name = ? and Cost < ?”; PreparedStatement pstmt = con.prepareStatement(queryStr); pstmt.setString(1, “t-shirt”); pstmt.setInt(2, 1000); ResultSet rs = pstmt.executeQuery();

17 Changing DB with PreparedStatement String deleteStr = “DELETE FROM Items " + "WHERE Name = ? and Cost > ?”; PreparedStatement pstmt = con.prepareStatement(deleteStr); pstmt.setString(1, “t-shirt”); pstmt.setInt(2, 1000); int delnum = pstmt.executeUpdate();

18 Statements vs. PreparedStatements: Be Careful! Are these the same? What do they do? String val = “abc”; PreparedStatement pstmt = con.prepareStatement(“select * from R where A=?”); pstmt.setString(1, val); ResultSet rs = pstmt.executeQuery(); String val = “abc”; Statement stmt = con.createStatement( ); ResultSet rs = stmt.executeQuery(“select * from R where A=” + val);

19 Statements vs. PreparedStatements: Be Careful! Will this work? No!!! A ‘?’ can only be used to represent a column value WHY? PreparedStatement pstmt = con.prepareStatement(“select * from ?”); pstmt.setString(1, myFavoriteTableString);

20 Timeout Use setQueryTimeOut(int seconds) of Statement to set a timeout for the driver to wait for a statement to be completed If the operation is not completed in the given time, an SQLException is thrown What is it good for?

ResultSet A ResultSet provides access to a table of data generated by executing a Statement Only one ResultSet per Statement can be open at once The table rows are retrieved in sequence –A ResultSet maintains a cursor pointing to its current row of data –The 'next' method moves the cursor to the next row

ResultSet Methods boolean next() –activates the next row –the first call to next() activates the first row –returns false if there are no more rows void close() –disposes of the ResultSet –allows you to re-use the Statement that created it –automatically called by most Statement methods

ResultSet Methods Type getType(int columnIndex) –returns the given field as the given type –fields indexed starting at 1 (not 0) Type getType(String columnName) –same, but uses name of field –less efficient int findColumn(String columnName) –looks up column index given column name

ResultSet Methods String getString(int columnIndex) boolean getBoolean(int columnIndex) byte getByte(int columnIndex) short getShort(int columnIndex) int getInt(int columnIndex) long getLong(int columnIndex) float getFloat(int columnIndex) double getDouble(int columnIndex) Date getDate(int columnIndex) Time getTime(int columnIndex) Timestamp getTimestamp(int columnIndex)

ResultSet Methods String getString(String columnName) boolean getBoolean(String columnName) byte getByte(String columnName) short getShort(String columnName) int getInt(String columnName) long getLong(String columnName) float getFloat(String columnName) double getDouble(String columnName) Date getDate(String columnName) Time getTime(String columnName) Timestamp getTimestamp(String columnName)

26 ResultSet Example Statement stmt = con.createStatement(); ResultSet rs = stmt. executeQuery("select name,age from Employees"); // Print the result while(rs.next()) { System.out.print(rs.getString(1) + ”:“); System.out.println(rs.getShort(“age”)+”“); }

Null Values In SQL, NULL means the field is empty Not the same as 0 or “” In JDBC, you must explicitly ask if a field is null by calling ResultSet.isNull(column) For example, getInt(column) will return 0 if the value is either 0 or null!!

28 Null Values When inserting null values into placeholders of Prepared Statements: –Use the method setNull(index, sqlType) for primitive types (e.g. INTEGER, REAL); –You may also use the setXXX(index, null) for object types (e.g. STRING, DATE).

29 ResultSet Meta-Data ResultSetMetaData rsmd = rs.getMetaData(); int numcols = rsmd.getColumnCount(); for (int i = 1 ; i <= numcols; i++) { System.out.print(rsmd.getColumnLabel(i)+” “); } A ResultSetMetaData is an object that can be used to get information about the properties of the columns in a ResultSet object. An example: write the columns of the result set Many more methods in the ResultSetMetaData APIResultSetMetaData API

Mapping Java Types to SQL Types SQL type Java Type CHAR, VARCHAR, LONGVARCHAR String NUMERIC, DECIMAL java.math.BigDecimal BIT boolean TINYINT byte SMALLINT short INTEGER int BIGINT long REAL float FLOAT, DOUBLE double BINARY, VARBINARY, LONGVARBINARY byte[] DATE java.sql.Date TIME java.sql.Time TIMESTAMP java.sql.Timestamp

31 More Information A detailed overview of type mapping and type conversion can be found at /mapping.html

Database Time Times in SQL are notoriously non-standard Java defines three classes to help java.sql.Date –year, month, day java.sql.Time –hours, minutes, seconds java.sql.Timestamp –year, month, day, hours, minutes, seconds, nanoseconds –usually use this one

33 Cleaning Up After Yourself Remember to close the Connections, Statements, PreparedStatements and ResultSets con.close(); stmt.close(); pstmt.close(); rs.close()

34 Dealing With Exceptions An exception can have more exceptions in it. catch (SQLException e) { while (e != null) { System.out.println(e.getSQLState()); System.out.println(e.getMessage()); System.out.println(e.getErrorCode()); e = e.getNextException(); }

35 Advanced Topics

36 LOBs: Large OBjects Two types: –CLOB: Character large object (a lot of characters) –BLOB: Binary large object (a lot of bytes) Actual data is not stored in the table with the CLOB/BLOB column. Only a pointer to the data is stored there I will show how to use a BLOB; CLOBs are similar

37 Retrieving a BLOB create table userImages( user varchar(50), image BLOB ); ResultSet rs = stmt.executeQuery(“select image from userImages”); while (rs.next) { Blob b = rs.getBlob(“image”); InputStream stream = b.getBinaryStream(); doSomething(stream); }

38 Inserting a BLOB PreparedStatement pstmt = con.prepareStatement(“insert into userImages values(‘snoopy’, ?)”); File file = new File(“snoopy.jpg”); InputStream fin = new FileInputStream(file); pstmt.setBinaryStream (1, fin, file.length()); pstmt.executeUpdate();

39 Transactions and JDBC Transaction = more than one statement which must all succeed (or all fail) together If one fails, the system must reverse all previous actions Also can’t leave DB in inconsistent state halfway through a transaction COMMIT = complete transaction ROLLBACK = abort

40 Example Suppose we want to transfer money from bank account 13 to account 72: PreparedStatement pstmt = con.prepareStatement(“update BankAccount set amount = amount + ? where accountId = ?”); pstmt.setInt(1,-100); pstmt.setInt(2, 13); pstmt.executeUpdate(); pstmt.setInt(1, 100); pstmt.setInt(2, 72); pstmt.executeUpdate(); What happens if this update fails?

41 Transaction Management Transactions are not explicitly opened and closed The connection has a state called AutoCommit mode if AutoCommit is true, then every statement is automatically committed if AutoCommit is false, then every statement is added to an ongoing transaction Default: true

42 AutoCommit If you set AutoCommit to false, you must explicitly commit or rollback the transaction using Connection.commit() and Connection.rollback() In order to work with LOBs, you usually have to set AutoCommit to false, while retrieving the data Note: DDL statements in a transaction may be ignored or may cause a commit to occur. The behavior is DBMS dependent setAutoCommit(boolean val)

43 Fixed Example con.setAutoCommit(false); try { PreparedStatement pstmt = con.prepareStatement(“update BankAccount set amount = amount + ? where accountId = ?”); pstmt.setInt(1,-100); pstmt.setInt(2, 13); pstmt.executeUpdate(); pstmt.setInt(1, 100); pstmt.setInt(2, 72); pstmt.executeUpdate(); con.commit(); catch (Exception e) { con.rollback(); }

44 Isolation Levels How do different transactions interact? Do they see what another has written? Possible problems: –Dirty Reads: a transaction may read uncommitted data –Unrepeatable Reads: two different results are seen when reading a tuple twice in the same transaction –Phantom Reads: tuples are added to a table between two readings of this table in a single transaction

45 Isolation Levels JDBC defines four isolation modes: Level Dirty Read Unrepeatable Read Phantom Read Read UncommitedYes Read CommitedNoYes Repeatable ReadNo Yes SerializableNo

46 Isolation Levels Set the transaction mode using setTransactionIsolation() of class Connection Oracle only implements: – TRANSACTION_SERIALIZABLE An exception may be thrown if serializability isn’t possible – TRANSACTION_READ_COMMITED This is the default

47 Level: READ_COMMITED Transaction 1: insert into A values(1) insert into A values(2) commit Transaction 2: select * from A Table: A 1 2 Question: Is it possible for a transaction to see 1 in A, but not 2? Question: Is it possible for the 2 queries to give different answers for level SERIALIZABLE?